The effect of air pollution on infant health is a topic of considerable interest. Previous research has documented a statistical association between differential pollution levels across sites and variation in adult health outcomes. However, a heated debate has arisen about whether these documented associations are causal. At issue is the possibility that because air pollution, and pollution changes, are not randomly assigned across localities, previous studies may not be adequately controlling for a number of potential confounding determinants of mortality. Because the ideal of randomized clinical trials is not feasible, the objective of the proposed project is to use an alternative approach which is similar in spirit. A solution to this evaluation problem is to use "quasi-experiments" in which forces outside of the researcher's control cause air pollution changes but do not independently affect infant health outcomes. The availability of events that cause differential changes in air pollution but are unrelated to unobserved determinants of infant health allows for unbiased inference on the relation between infant mortality and pollution. The project will use two quasi-experiments: 1) differential air quality changes across sites attributable to geographic variation in the effects of a recession, and 2) differential pollution changes induced by federal air pollution regulations imposed at the county level. The analysis compares changes in infant mortality rates in counties that had large pollution reductions caused by the exogenous event to changes in counties with no pollution reductions to identify the pollution-infant mortality relation. Also, the analysis is based on the most detailed and comprehensive data available on air pollution, pollution regulations, infant births and deaths, and other potential determinants of infant health outcomes. The proposed research designs applied to the array of data sources that have been collected provide a unique opportunity for disentangling the relationship between pollution and infant health from a number of potential confounding factors. This research program should yield an improved understanding of the health consequences of air pollution.